In the operation of internal combustion engines, air is supplied to the exhaust gas of the engine after a start. The exothermal reaction of the air, which is blown in between the outlet valves of the engine and the catalytic converter, and the hot exhaust gas and the further oxidation thereof before and in the catalytic converter leads to an accelerated warming of the catalytic converter to its operating temperature and therefore to a reduction of the emission of toxic substances in the warm-up phase of the engine.
Statutory requirements in the United States provide for monitoring systems for blowing in secondary air so that a minimum air flow is supplied.
A method and arrangement for diagnosing secondary air systems is disclosed in U.S. Pat. No. 5,444,975. According to the teaching in this patent, the reaction of the exhaust-gas probe (or of the lambda control, which is coupled to the exhaust-gas probe, for adjusting the composition of the air/fuel mixture for the engine) is evaluated as to the activation of an electrically driven secondary air pump. An intact secondary air system leads to an excess of oxygen in the exhaust gas. Especially for high capacity secondary air pumps, the problem is present that the supply of a large quantity of secondary air triggers an intense enrichment of the air/fuel mixture by the lambda control loop. The quantity of the emitted toxic substances then increases dramatically because the engine burns a very rich mixture. The running performance of the engine can, under some circumstances, deteriorate significantly. In order to keep these disadvantages at a minimum, the known method provides for an adaptation of the supplied secondary-air quantity to the value of the exhaust-gas quantity, which is supplied by the engine, by varying the pumping capacity of the secondary-air pump. For this purpose, means are provided which make it possible to drive the secondary-air pump in at least two stages in dependence upon operating parameters of the engine.